Optical defects in lenses: The defect in which white light from a point source is dispersed into component colors that focus at different distances, producing a blurred, colored image, is called:

Introduction / Context:
Surveying telescopes and cameras rely on high-quality optics. Different lens aberrations degrade image quality in distinct ways. Recognizing them helps with instrument selection, maintenance, and understanding the limits of visual observations in the field.

Given Data / Assumptions:

• White light enters a refracting optical system.
• The defect causes color fringing and loss of sharp focus.
• We consider common, classical aberrations in lenses.

Concept / Approach:
Chromatic aberration arises because a lens’s refractive index varies with wavelength (dispersion). Blue light bends more than red, so different colors have different focal lengths. As a result, a polychromatic point source forms colored halos and cannot be sharply focused simultaneously in all colors, unless achromatic combinations are used. This is distinct from spherical aberration (marginal rays focusing differently from paraxial rays), astigmatism (different foci in tangential and sagittal planes), and coma (comet-like flares for off-axis points).

Step-by-Step Solution:

Verification / Alternative check:
Examine a high-contrast edge through the instrument; color fringes (purple/green) indicate chromatic aberration. Stopping down the aperture reduces marginal ray effects but does not remove chromatic dispersion entirely.

Why Other Options Are Wrong:

• Spherical aberration: Colorless blur due to spherical surfaces, not wavelength dispersion.
• Astigmatism: Produces line focus differences, especially off-axis.
• Coma: Off-axis points form comet-shaped flares, typically colorless.

Common Pitfalls:
Confusing colored fringes (chromatic) with general softness (spherical aberration). Also, don’t attribute atmospheric dispersion (near horizon) to lens flaws.

Final Answer:
Chromatic aberration